JP2002354276A - Method and device for converting color - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide color conversion method and device capable of improving the accuracy of color conversion while suppressing the increase of capacity for storing a multi-dimensional LUT. SOLUTION: The color conversion method for describing the relation of chrominance signal values obtained before and after conversion and performing color conversion by using the LUT and interpolation converts colors by calculating each output value from the grating point of a main multi-dimensional LUT in an area where there is no sub-multi-dimensional LUT and from the grating point of the sub-multi-dimensional LUT in an area where there is the sub-multi- dimensional LUT by using the main multi-dimensional LUT for dividing a range including the chrominance signal value of an input and storing an output value corresponding to the input value of the grating point and the sub-multi- dimensional LUT for dividing an area surrounded by the grating points of the main multi-dimensional LUT and storing an output value corresponding to the input of the grating point and the color conversion device is provided with means suitable for the method.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention describes the relationship between a color signal value before conversion and a color signal value after conversion in a color conversion using a multidimensional look-up table, and uses the look-up table and an interpolation operation. The present invention relates to a method for performing color conversion.

[0002]

2. Description of the Related Art Conventionally, the relationship between two color spaces has been described using a multidimensional look-up table (in the present specification, a look-up table is abbreviated as an LUT), and a color is obtained by the LUT and an interpolation operation. Methods for performing the conversion have been implemented. In general, color space conversion is often a complicated non-linear conversion, and a method using a multidimensional LUT is one of effective means. The multidimensional LUT is stored on a grid point corresponding to an input signal value in a format for holding an output signal value.

FIG. 5 is a diagram for explaining the relationship between grid points of a multidimensional LUT and output values stored therein. At the grid points corresponding to the color signal values (X2, Y2, 0) in the input color space, the color signal values (α, β, γ) in the output color space are held. That is, in this color conversion, the input values (X2, Y2,
0) is converted to an output value (α, β, γ). Although only one output value is shown in the figure, all grid points actually have output values.

[0004] In this example, three-dimensional to three-dimensional conversion is shown, but the number of dimensions is not limited for both input and output. If a multidimensional LUT can have grid points corresponding to all combinations of values that can be taken as inputs, color conversion can be performed accurately without performing interpolation calculations. For example, if the input color space is CMYK and 1 Assuming that all combinations have% intervals, the total number of grid points is 10
An enormous number of 1 ⁴ = 104060401 points, and an enormous storage capacity is required to store this multidimensional LUT. Therefore, in the multi-dimensional LUT, the input color signal is divided at regular intervals, and only the grid points corresponding to the divided color signals are stored. The method of finding the value is adopted.

As described above, if the input color space is CMYK and the color signal division is performed at 10% intervals, the number of grid points that need to be stored is 11 ⁴ = 14641. In this case, for an input value without grid points, for example, 15%, 10%
An output value is obtained from the interpolation calculation using the values on each grid point of 20%.

In this method, the accuracy of color conversion is greatly affected by the interval of division of the color space. If the number of divisions of each axis of the color space is small and the relationship between input and output is a highly nonlinear relationship, an error that occurs during interpolation calculation becomes large, and the accuracy of conversion is reduced. However, when the number of divisions of each axis is increased in the multidimensional LUT, the storage capacity required to store the LUT rapidly increases, and the accuracy of color conversion using the multidimensional LUT can store the multidimensional LUT. There is a problem that the storage capacity is limited.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art, and can improve color conversion accuracy while suppressing an increase in storage capacity for storing a multidimensional LUT. It is an object to provide a color conversion method and a color conversion device.

[0008]

Means for Solving the Problems To achieve the above-mentioned object, a means provided by the present invention firstly stores a multi-dimensional look-up table for a relationship between color signal values before and after conversion. In the method of performing color conversion by using the look-up table and the interpolation operation, the space of the input color signal value is divided, and an output value corresponding to an input value corresponding to a grid point formed according to the division is obtained. The retained main multidimensional look-up table and the sub-multiplier storing an output value corresponding to an input corresponding to a grid point formed according to the division by further dividing a region surrounded by grid points of the main multidimensional look-up table By using the dimensional lookup table, in an area where the sub-multi-dimensional lookup table does not exist, from the grid points of the main multi-dimensional lookup table,
Further, in a region where a sub-multidimensional lookup table exists, a color conversion method is characterized in that output values are calculated from grid points of the sub-multidimensional lookup table to realize color conversion.

Here, for the main multidimensional LUT, dividing the input color space at regular intervals is generally simpler and easier to understand, and the calculation of the addresses at which the grid points are stored is easier. Is preferred. But,
If the input color space division interval of the main multidimensional LUT is not fixed, it may be. In this case, for example, the present invention can be applied by taking measures such as preparing a mechanism capable of calculating an address at which a grid point is stored.

According to a second aspect of the present invention, there is provided an apparatus for describing a relationship between color signal values before and after conversion using a multidimensional look-up table, and performing color conversion by using the look-up table and an interpolation operation. An input means for inputting a color signal value to be converted, the input color signal value space is divided, and an output value for an input value corresponding to a grid point formed according to the division is defined as a main multidimensional lookup table. Main multidimensional look-up table storage means for holding, further dividing a region surrounded by grid points in the main multidimensional look-up table, and outputting an output value corresponding to an input value of a grid point generated according to the division to a sub-multiple A sub-multi-dimensional look-up table storing means for storing as a dimensional look-up table, an area surrounded by grid points of the main multi-dimensional look-up table, Sub-multi-dimensional lookup table management data storing means for storing sub-multi-dimensional lookup table management data describing a correspondence relationship with a lookup table; a multi-dimensional lookup table corresponding to a color signal value input by the input means Multi-dimensional lookup table reference means for determining a reference location, and multi-dimensional lookup table selecting means for determining a multi-dimensional lookup table to be used for determining an output value by referring to the sub multi-dimensional lookup table management data Using the reference location determined by the multidimensional lookup table reference means and the information of the multidimensional lookup table selected by the multidimensional lookup table selection means, the main multidimensional lookup table storage means or the sub multidimensional lookup table Stored in one of the lookup table storage means From a multidimensional lookup table, the output value determining means for determining a color signal value of the output, which is the color conversion device characterized by having.

Here, it is generally simpler and easier to understand that the input color space is divided at regular intervals for the main multidimensional LUT, and the calculation of the address where the lattice point is stored is also simple. Is preferred. But,
If the input color space division interval of the main multidimensional LUT is not fixed, it may be. In this case, for example, the present invention can be applied by taking measures such as preparing a mechanism capable of calculating an address at which a grid point is stored. The term “multi-dimensional lookup table” in the claims refers to a group of lookup tables including a main multi-dimensional lookup table and one or more sub-multi-dimensional lookup tables.

[0012]

FIG. 1 is a block diagram showing an example of a color conversion apparatus according to the present invention. In the figure, 1 is an input device such as a keyboard, 2 memories, 3 is a CPU, 4 is an input / output interface, 5 is an external storage device, and 6 is a CRT.
Such as a monitor.

An input device 1 such as a keyboard is used to input an input color signal to be converted or to select various data recorded in a file format in an external storage device or the like. The memory 2 stores, in addition to the input color signals to be converted, the multidimensional LUT read from the external storage device,
This is for holding variables and the like required in the processing by PU3.

The CPU 3 controls each device based on the function of software and performs the following processing.

1) An address necessary for referring to a multidimensional LUT is generated from an input value to be converted.
2) Referring to the sub-multidimensional LUT management data, it is determined whether or not the area surrounded by the grid points used when converting the input value has the sub-multidimensional LUT. 3) Main multidimensional L
The output value after conversion is calculated by performing an interpolation operation on the data held in the UT or the sub-multidimensional LUT.

The external storage device 5 is a readable and writable storage device, and holds a multidimensional LUT necessary for conversion, and holds a conversion result. The monitor 6 displays the progress of the conversion, the conversion result, and other necessary information.

Next, an example of the processing of this embodiment will be described with reference to the flowchart of FIG. In this embodiment, the input color signal is described as RGB and the output color signal is described as XYZ, but the color signal space is not necessarily limited, and any color space may be used. For example, CM
YK, CIELAB, and the like. Also, CMY
The color signal unique to the device, such as K, RGB, etc., may indicate any amount such as a dot area ratio, an ink amount, a toner amount, or a drive voltage, a device, or the like.

In this embodiment, a number is regularly assigned to a small area surrounded by lattice points of the main multidimensional LUT, and sub-multidimensional LUT management data describing the relationship between the number and the number of the sub-multidimensional LUT. An array having the data structure shown in FIG. 4 as an element is used. FIG. 4 also shows an example of region numbering.

(1) Reading of multi-dimensional LUT [101 of flowchart] The main multi-dimensional LUT, sub-multi-dimensional LUT group, and sub-multi-dimensional LUT management data stored in the external storage device 5 are stored in the memory 2 via the interface 4. Read in. (2) Reading the color signal to be converted [1 in the flowchart]
02] A file in which a color signal to be converted is recorded is read from an external storage device to the memory 2 via the interface 4.

(3) Calculating the reference position of the main multidimensional LUT [103 in the flowchart] The input color signal value and the main multidimensional LUT read into the memory
From the number of divisions, the address in the memory where the lattice point to be referred to when calculating the output value is stored is calculated. If the input value is not a value that matches the lattice point of the multidimensional LUT, all the addresses of the neighboring lattice points required for the interpolation calculation are calculated. In this description, it is assumed that a main multidimensional LUT is obtained by dividing an input color space at regular intervals.

(4) Judgment of existence of sub-multidimensional LUT and calculation of output value [104 to 106 in flowchart] Referring to the sub-multidimensional LUT management data, the subspace containing the input color signal to be converted is sub-multiple. It is determined whether or not it has a dimensional LUT. If the sub multidimensional LUT does not exist, the output value is calculated with reference to the value described in the main multidimensional LUT. If a sub-multidimensional LUT exists, the output value is calculated with reference to the sub-multidimensional LUT indicated by the sub-multidimensional LUT management data. Generally, this calculation is realized in the form of a weighted average for neighboring grid points, but any other method may be used for the interpolation method.

(5) Saving and Displaying the Result [107 in the Flowchart] The color signal value in the converted color space obtained as a result of the conversion is displayed on the monitor 6. The result is recorded in the external storage device 5.

The embodiment of the present invention has been described above.
In addition, the following modified examples that do not depart from the gist of the present invention can be considered.

(1) An input value of a color signal to be converted may be directly input from an input device such as a keyboard. (2) A program for generating an arbitrary color signal in the color space on the input side may be run on the CPU, and the data may be input.

(3) A multi-dimensional LUT obtained by further dividing a region surrounded by lattice points of the sub-multi-dimensional LUT is retained. The multi-layer LUT is not limited to the two-layer structure of the main multi-dimensional LUT and the sub-multi-dimensional LUT. May be built. (4) Main multi-dimensional LUT and sub-multi-dimensional LUT accompanying it
The group and sub-multidimensional LUT management data may be stored as one file instead of separate files.

[0026]

According to the color conversion method of the present invention, by using a sub-multi-dimensional LUT group obtained by further dividing a part of the multi-dimensional LUT used for the color conversion, the capacity for storing the multi-dimensional LUT is reduced. It is possible to improve the color conversion accuracy of an arbitrary region in the color space without greatly increasing the color conversion accuracy. As a result, it is possible to improve the color conversion accuracy of a color region where higher accuracy is required or a color region where a color difference is easily perceived by human eyes.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating an example of a color conversion device according to the present invention.

FIG. 2 is a functional block diagram illustrating functions according to an example of the color conversion apparatus according to the present invention.

FIG. 3 is a flowchart illustrating an example of processing of a color conversion device according to the present invention.

FIG. 4 is a diagram illustrating an example of sub multidimensional LUT management data.

FIG. 5 is a conceptual diagram illustrating the structure of a multidimensional LUT.

FIG. 6 is an explanatory diagram showing a relationship between a main multidimensional LUT and a sub multidimensional LUT.

[Explanation of symbols]

 1. Input device such as keyboard 2. Memory 3. CPU 4. Interface 5. External storage device 6. Monitor

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Takayuki Hasegawa 1-1-1, Taito, Taito-ku, Tokyo Toppan Printing Co., Ltd. (72) Inventor Seishi Furuya 1-1-1, Taito, Taito-ku, Tokyo Letterpress F term (reference) in Printing Co., Ltd.

Claims (2)

[Claims] 1. A method of describing the relationship between color signal values before and after conversion using a multidimensional look-up table and performing color conversion by using the look-up table and an interpolation operation. A main multidimensional look-up table holding output values for input values corresponding to grid points formed according to the division, and further dividing a region surrounded by the grid points of the main multidimensional look-up table, By using a sub-multidimensional look-up table that holds output values for inputs corresponding to grid points formed according to the division, in a region where the sub-multidimensional look-up table does not exist,
Calculating the output values from the grid points of the main multidimensional look-up table and from the grid points of the sub-multidimensional look-up table in the region where the sub-multidimensional look-up table exists, to realize color conversion. Characteristic color conversion method. 2. An apparatus for describing the relationship between color signal values before and after conversion using a multidimensional look-up table and performing color conversion by using the look-up table and an interpolation operation, wherein the color signal value to be converted is A main multidimensional lookup table that divides a space of input color signal values and holds output values for input values corresponding to grid points formed according to the division as a main multidimensional lookup table A storage unit that further divides an area surrounded by grid points in the main multidimensional lookup table, and stores an output value corresponding to an input value of a grid point generated according to the division as a sub multidimensional lookup table; Multidimensional lookup table storage means, Correspondence between an area surrounded by lattice points of the main multidimensional lookup table and the sub multidimensional lookup table A sub-multi-dimensional look-up table management data that describes the relationship, a sub-multi-dimensional look-up table management data storage unit that holds the multi-dimensional lookup table management data that corresponds to the color signal value input by the input unit; Multi-dimensional lookup table reference means for determining; multi-dimensional lookup table selecting means for referring to the sub multi-dimensional lookup table management data to determine a multi-dimensional lookup table to be used for determining an output value; multi-dimensional lookup Main multidimensional look-up table storage means or sub-multidimensional look-up table storage means using the reference location determined by the table reference means and the information of the multidimensional lookup table selected by the multidimensional look-up table selection means. Multi-dimensional lookup stored in one of The color conversion apparatus from Buru, the output value determining means for determining a color signal value of the output, characterized by having a.